1. Technical Field
The present invention relates to a process and apparatus for making photosensitive resin printing plates and, more particular, to a Process for making photosensitive resin printing plates which have a continuous and reliable high plate thickness and can produce printed matter with good ink coverage and little dot gain, such as photosensitive resin printing plates for relief printing, and as in corrugated board printing plate-making apparatus used therefor.
2. Discussion of Related Art
Plate making processes which use a liquid photosensitive resin to make relief plates for printing corrugated boards, newspapers and so forth are in increasingly wide use because, as compared with prior rubber printing plate making, they produce printing plates having excellent printing quality with less process steps.
The above-mentioned printing plate making which uses a liquid photosensitive resin is generally performed in the following manner. First, as shown in FIG. 4, a negative film 2 and a cover film 3 are placed one upon the other between two transparent substrates 1 and 1'. A liquid photosensitive resin layer 4 is formed thereon, and a base film 5 and a masking film 6 are laid thereon, one upon the other. Actinic light from light sources 7 and 7' is irradiated from both sides thereof to form images of relief parts and base parts. Next, the masking film 6 is removed, and back exposure is performed for fixing the relief parts formed on the base film 5. Numerals 8 and 8' indicate sponge dams for preventing the overflow of the liquid photosensitive resin.
Another method can also be used in which the masking film is not used and a back exposure for forming a relief base on the whole surface is performed from the base film side and then a relief exposure for forming the relief image is conducted from the negative film side.
Subsequently, in either case, developing is performed by washing out the uncured parts with a suitable washing agent and then post-exposure and drying treatments are applied, whereby the intended printing plate is obtained.
Such plate-making processes, however, incur the following disadvantages. As is apparent from the flow chart of the prior plate-making process shown in FIGS. 5A to 5D, when exposure is performed from the side of the upper transparent substrate 1', the upper substrate 1' undergoes deformation owing to the heat of polymerization generated by the photosensitive resin, whereas when exposure is performed from the side of the lower transparent substrate 1, the lower substrate 1 undergoes deformation in the same manner, so that the resulting printing plate has a large difference in thickness between the central part of the plate and the peripheral parts thereof, as shown in the sectional view of FIG. 6. When printing is performed by using such a printing plate, it is difficult to obtain clear printed images at the central part thereof because Poor ink coverage is experienced owing to the small plate thickness, while printed images with lower dot gain at the peripheral parts are obtained owing to the higher plate thickness.